This invention relates to detectors for detecting impedance mismatches between transmission lines and load impedances. More particularly, the invention relates to a detector which is capable of processing not only reflection coefficient magnitude information but also reflection coefficient phase information as a basis for determining the existence of a mismatch.
In radio electronics, it is often helpful to know whether a transmission line is matched or mismatched to a load impedance. A directional coupler circuit such as that shown in FIG. 1 may be used to make this determination. This example of a directional coupler is one of the components in the Antenna Tuner Discriminator of U.S. Pat. No. 4,493,112 issued to Warren B. Bruene on Jan. 8, 1985. The elements of this coupler circuit are selected such that when the RF transmission line through the coupler is terminated in a proper load impedance, for example 50 ohms, the voltage drop across resistor R1 is equal to and in phase with the voltage across capacitor C2 of the voltage divider C1-C2. When properly terminated as above, the forward voltage, V.sub.f, is a positive value. At the same time, the voltage drop across resistor R2 is equal to but opposite in phase with the voltage drop across capacitor C2. Since the termination is proper, there will be no reflected voltage wave. In other words, the voltage of the reflected wave, V.sub.r, will have a magnitude of zero. However when the impedance of the load is not the proper termination resistance, 50 ohms in the example, a reflected voltage wave will be generated. Thus, V.sub.r will have a non-zero magnitude. In the present example, the resistances of resistors R1 and R2 are equal and substantially less than the impedance of inductor L.
Those skilled in the art appreciate that the above directional coupler can be used as a detector which determines when a load impedance is mismatched to a transmission line. The extent of the mismatch is indicated by the magnitude of the reflected voltage wave, V.sub.r. As the mismatch becomes larger, the reflected voltage wave becomes larger and, correspondingly, the voltage standing wave ratio (VSWR) becomes larger.
As already mentioned, the above discussed directional coupler can be used to indicate when a mismatched load impedance is present. However, it does not give any information about the specific nature of the load impedance, that is, whether it is capacitive, inductive or resistive or a combination thereof. Stated alternatively, the directional coupler of FIG. 1 gives no phase information about the impedance under test.
Turning now to FIG. 2, a Smith Chart representing load impedance is shown. In general, this Smith Chart represents all impedances which the load impedance could possibly assume. Those skilled in the art use Smith Charts to plot load impedances in a manner which indicates the extent to which such impedances are resistive, capacitive, inductive, or combinations thereof.
It is noted that load impedances in specific ranges are most likely to cause oscillation in a given RF power amplifier. The Smith Chart of FIG. 2 includes a cross-hatched region 10 which is defined to be one such region of instability for purposes of this example. That is, region 10 of the Smith Chart represents a range of different values of load impedance which would cause an amplifier coupled to such impedance to become unstable. Portions of the Smith Chart generally above and to the right of region 10 form a region 20 which represents load impedances resulting in stable amplifier operation. An seen in FIG. 2, region 20 is generally circular in shape and includes a center 22. The edge of region 20 which is bounded by unstable region 10 is referred to as threshold circle 24 because circle 24 represents the threshold between stable and unstable amplifier operation. Center 22 is also referred to as the threshold center.
Prior to the detector of the invention, mismatch detectors could determine whether or not a particular load impedance was inside or outside of a threshold circle centered at the Smith Chart origin, such as threshold circle 30 (discussed later). However, such conventional detectors did not address the problem of determining whether or not a load impedance is inside or outside a threshold circle centered at a point on the Smith Chart other than at the origin, such as threshold circle 24.
It will be recalled that the directional coupler discussed above merely determines the magnitude of the reflected voltage signal resulting from a particular load impedance. The coupler can thus be used to determine if a particular load impedance results in a VSWR which exceeds a predetermined value. In other words, the coupler is used to determine whether or not a given load impedance is inside or outside of a threshold circle centered at the origin such as the 2.5 to 1 VSWR threshold circle 30. The center or origin of the Smith Chart represents a perfectly matched load. As we move away from the center of the Smith Chart in any direction, the extent of the mismatch and the magnitude of the VSWR increase. By way of example, all the points on the threshold circle 30 represent different mismatched load impedances which would cause a VSWR of 2.5 to 1. It is noted that the 2.5 to 1 VSWR threshold circle 30 is tangent to the cross-hatched region of instability 10. It is again noted that any mismatch will result in some VSWR, however many impedance mismatches may not cause instability problems. In many situations, only a certain range of load impedances, such as cross-hatched region 10, will cause instability. As pointed out in the subsequent discussion, a conventional directional coupler detector circuit is of limited application under these circumstances because it can not determine whether or not a certain impedance lies within or without a threshold circle having a center other than at the center of the Smith Chart.
With respect to FIG. 2, it is noted that the cross-hatched region of instability 10 is tangent to circle 30. Further, note that circle 30 is centered at the Smith Chart origin. Assume now that the directional coupler of FIG. 1 is used to detect when the load impedance becomes so high or low that such load impedance is not within circle 30. Stated alternatively, such directional coupler is used to determine when the load impedance causes a predetermined VSWR (eg. 2.5 to 1) to be exceeded. The occurrence of this condition can be used to trigger appropriate amplifier stabilization circuitry. For a circuit which is capable of achieving stabilization in this manner, see U.S. Pat. No. 4,439,741 entitled Stabilized High Efficiency Radio Frequency Amplifier, issued to Harvey N. Turner, Jr. on Mar. 27, 1984 and assigned to Motorola, Inc., the assignee of the invention described herein. The disclosure of U.S. Pat. No. 4,439,741 is incorporated herein by reference.
By examining the Smith Chart of FIG. 2, it is clear that if the directional coupler of FIG. 1 were used as the mismatch detector, stabilization circuitry would have to be activated at every point outside of threshold circle 30 to insure amplifier protection in the cross-hatched region of instability 10 tangent thereto. In some applications, such an arrangement may be undesirable and inefficient because the stabilization circuitry, which consumes power, would be activated for many load impedance values where it is really not necessary, namely those load impedance values outside of the cross hatched region of instability 10 and not within circle 30. This problem is encountered because prior mismatch detectors were limited to determining whether or not a particular load impedance was within or without a threshold circle centered at the origin of the Smith Chart. In contrast to prior mismatch detectors, the present invention is directed to detecting the presence of load impedances within a range of values outside a threshold circle having a center on the Smith Chart which may be other than the normalized origin of the Smith Chart.
One impedance mismatch detection system which employs the impedance mismatch detector of the present invention is described and claimed in the copending United States patent application entitled Adaptive Impedance Mismatch Detector System, Ser. No. (to be assigned), filed (to be assigned), such application having the same inventor and assignee as the present invention.
The features of the invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to structure and method of operation, may best be understood by reference to the following description considered together with the accompanying drawings.